Stainless steel
The workhorse. Inert, unbreakable in normal use, and the only material that can be perforated finely enough to make a genuinely good infuser. It is rarely anyone's favourite and it is the correct answer more often than any other material here.
What it is
An alloy of iron with chromium, uniform in atomic structure rather than polycrystalline, which is why it deforms instead of shattering. Not every metal works for teaware: some react with tea compounds and change the taste, some are toxic, and a few melt below the boiling point of water. Chromium's contribution is the passive oxide layer that stops the iron corroding.
What it does to your tea
Nothing chemically. It is durable and unreactive, and unlike plastics it leaves no particles behind. Its real advantage is mechanical: steel can be laser-cut with perforations fine enough to keep out all but the smallest fragments of leaf, which glass cannot match, and the large well-perforated steel basket is the closest thing to loose-leaf brewing you can get without a dedicated pot. Thermally it conducts far better than ceramics, so a steel cup gets hot to hold and a steel vessel cools its contents faster than a ceramic one of the same mass.
What it suits
Infusers and strainers above all, and kettles, and travel flasks where the double wall does the insulating. It is a poor choice for a cup you hold, for the same reason it is a good choice for a kettle.
Care and cleaning
Dishwasher, detergent, scouring pads within reason. Rinse infusers before the leaf dries in the mesh, which is the only real maintenance it needs. Discoloured steel from hard water descales with citric acid.
What people get wrong
Thinking metal always metallicises tea. That is true of some metals and specifically not of stainless steel, which is why it is the standard for infusers and kettles. The metallic taste people attribute to it usually comes from an unlined iron vessel, an aluminium one, or a badly plated novelty item, not from steel.
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